For a conventional OLED pixel circuit, a driving current flowing through an OLED is controlled through a driving transistor operating at a saturated region. During the operation of the pixel circuit, a data voltage signal of a data line is written into a gate electrode of the driving transistor under the control of a gate line, and then a corresponding driving current signal is outputted in accordance with an output characteristic of the driving transistor at the saturated region, so as to drive the OLED to emit light with a corresponding grayscale brightness. The grayscale brightness of the OLED in the conventional pixel circuit is directly determined by the data voltage signal of the data line, so it is difficult to provide larger grayscale brightness.